Hit list: Most vulnerable Senate Dems

National Republican Senatorial Committee Chairman John Cornyn has consistently insisted that regaining control of the upper chamber would be a two-cycle process. With the party committees already busy firing rockets at prospective opposition candidates, the second half of his timeline is under way.

Democrats have 23 seats to defend in two years and as many as nine seem potentially competitive. With the GOP just four seats short of its goal, POLITICO pinpoints the five Democrats currently at the top of the Republican target list.

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Nebraska Sen. Ben Nelson

It took all of three days after the election for the first Republican to throw his hat in the ring against the two-term Nelson. Attorney General Jon Bruning took the initial step of forming an exploratory committee after popular GOP Gov. Dave Heineman signaled he would stay put as chief executive through 2012.

“I’m not a guy who plays games. I didn’t think there was any advantage to playing coy,” Bruning told POLITICO.

Nelson, who cruised to reelection by 28 percentage points in 2006 despite being outraised 2-to-1, is seen as considerably more vulnerable now because of his vote for the stimulus and his role in the health care reform debate.

A poll released by the Nebraska Republican Party this week showed Nelson’s favorable rating right at 50 percent but also found him trailing the 41-year-old Bruning in a hypothetical matchup by 15 points. Nelson is making all the moves of a candidate, raising money and holding events just this week to defend the stimulus.

But the former governor hasn’t issued a declarative statement about pursuing a third term.

“It’s too early to be talking about 2012,” said Nelson spokesman Jake Thompson. “He’s said he’s planning to run, but a final decision will be made at the appropriate time.”

Bruning, who anticipates a competitive primary, said he expects Nelson to be the Democratic standard bearer. “I’m certainly preparing for him to run, and he should be prepared to be targeted,” he said. State treasurer-elect Don Stenberg, whom Nelson defeated to win the seat in 2000, is also pondering a bid.

Montana Sen. Jon Tester

The flat-topped junior senator from Montana starts the cycle vying with Nelson for the undesirable status of most endangered Senate Democrat. On Saturday, Bozeman businessman Steve Daines is expected to enter the race, touting his success as a local job creator at an international consulting company. The Montana Democratic Party is welcoming Daines to the fray with a Federal Election Commission complaint that accuses him of using “soft money” to air an attack ad aimed at Tester that is masked as issue advocacy.

The big question mark in the race is the state’s at-large Congressman Denny Rehberg, who hasn’t yet indicated his plans. His spokesman would only go as far as to say, “Denny is focused on doing the job the overwhelming majority of Montana voters sent him to Washington to do.”

State GOP Chairman Will Deschamps suggests that if Rehberg wants the nomination, it’ll be his for the taking.

“Last two cycles, Denny has carried Missoula County, which is so blue you can’t even think straight,” said Deschamps.

Neil Livingstone, a frequent national security commentator on Fox News Channel, told POLITICO he’s also eyeing the contest but said that if Rehberg runs, he won’t.

“Denny’s a good friend of mine, and I’ll support him on his decision,” he said.

Virginia Sen. Jim Webb

Whether or not Webb runs for a second term, Virginia Republicans have made it clear already that they’re targeting this seat — and for good reason.

The state’s political landscape has changed dramatically since Webb first won office in 2006, with Republicans sweeping the statewide offices in 2009 and picking up three House seats one year later. Webb, a former secretary of the Navy, has not committed to running for another term, and his spokesman told POLITICO that Webb “will address his intentions regarding the 2012 election cycle after giving the matter proper thought and consideration.”